Leadless lead frames are commonly used to package semiconductor devices. A leadless lead frame includes a plurality of die attach pads arranged in an array. Leads are provided around each of the die attach pads. Tie bars are used to hold the die attach pads and the contacts together. During the packaging process, a die is attached to each die attach pad using an epoxy or other adhesive. Wire bonds are then formed between the dice and the adjacent leads respectively. The surface of the lead frame with the dice attached thereon is then encapsulated, typically with an epoxy, using a transfer mold. Once the epoxy cures, the leaded frame is mechanically buffed to remove any mold compound flash or bleed. A mask printing is then performed to isolate the traces on the backside of the package that could potentially cause an electrical short on the PCB or to isolate areas on the exposed DAP to improve solder the uniformity of solder deposition and subsequent solder reflow. A solder plate is then applied to the undersurface of the die attach pad. The solder paste is used to later attach the package to a substrate such as a printed circuit board after it has been signulated from the lead frame. The individual packages are singulated by cutting the lead frame along the tie bars.
A number of problems are associated with the process of attaching the singulated packaged devices to a substrate. When the solder paste is applied to the undersurface of the die attach pads, it is usually dispensed as a ball at the center of the pad. During the attachment process when the solder is reflowed, the material tends to flow toward the periphery of the pad. As a consequence, the solder paste tends assume an “dome” shape. The height of the dome formed on the exposed DAP is problematic because it is typically higher than the “dome” formed on the input/output pads of the chip. As a consequence, board mounting can be very difficult. This uneven heights of the two domes tends to complicate the process of attaching the package to the substrate. To alleviate this problem, a solder paste stencil design for each and every type of package size and configuration is required will have to ensure the uniform height of solder bumps on the package. This process is not only tedious, it is difficult to optimize. Each package type will require different amounts of solder paste, depending on the size of the die attach surface and other factors.
In other applications, adhesive materials have been used to isolate exposed parts of the lead frame. Ideally only the input/output solder pads should be exposed on the backside of the package. All other features should be insulated to minimize the possibility of electrical shorting.
An apparatus and methods for using adhesive materials as insulating coatings for leadless lead frame semiconductor packages is therefore needed.